1. Field of the Invention
The present invention is directed to reaction products of Michael addition donor(s) and monoacrylate(s), the reaction product including at least one photolabile group, photopolymerizable compositions including the same, and methods related thereto.
2. Brief Description of Related Technology
Radiation curable adhesives and coatings containing (meth)acrylate monomers and oligomers and other free-radically polymerizable materials are widely used in applications where the cured material may come into contact directly or indirectly with food, drugs or medical devices. In such applications, it is desirable to ensure, to the extent that toxic or otherwise hazardous compounds are used in such products, that migration, evaporation or extraction of such materials from the cured material is controlled or ameliorated. A particular source of contamination in UV/visible light-cured products has been identified as residual unreacted photoinitiators and their decomposition products, such as benzaldehydes. Consequently, most commercially available photoinitiators are prohibited from use in such applications, examples of which include foil-laminating adhesives for food packaging, coil coatings for kitchen appliances and certain drug delivery/packaging systems.
For certain applications, users have employed electron beams (EB) instead of photoinitiator additives to cure such formulations. EB radiation is readily adsorbed by (meth)acrylate monomers and has sufficient photonic energy to initiate curing directly without the need for added photoinitiators. However, EB equipment is extremely expensive and can be used and justified only in certain applications in which production cost is not a primary consideration. Consequently, there is a need to develop UV and visible light curing additives and systems that produce cured materials having low extractables.
U.S. Pat. Nos. 5,945,489 and 6,025,410, U.S. Patent Application Publication No. 2005/0080162 and European Patent Application No. EP 1 342 737 disclose the reaction of an acetoacetate compound with an excess of a multi-functional acrylate to give an acrylated terminated oligomer containing a β-diketonic group incorporated into the polymer backbone. U.S. Pat. No. 6,673,851 and U.S. Patent Application Publication No. 2003/0073757 disclose subsequent reaction of such oligomers with a primary and/or secondary amines.
These oligomers undergo rapid UV initiated polymerization without the need for added photoinitiator, have good properties and are claimed to be suitable for flexible packaging adhesives, flexographic inks and screen-printing applications.
The process is, however, difficult to control at a manufacturing level and generally can result in non-uniform compositions with a variable content of gel particles. The reaction involves the stepwise addition of a multi-functional Michael donor (e.g. acetoacetate derivative) with a stoichiometric excess of a multifunctional Michael acceptor (e.g. diacrylate or polyacrylate) in the presence of a basic catalyst (e.g. tetramethylguanidine). The procedure can require care to avoid gelation, which can occur if the ratio of the equivalents of donor and acceptor groups, r, exceeds a value such that the fractional conversion required for gelation, alpha (α), is equal to or less than one, according to Equation 1:r=1/α2(fa−1)(fb−1)  Equation 1where fa and fb are the functionalities of the donor and acceptor groups. See C. Macosko and D. Miller, Macromolecules 1976, 9(2), 199. When the ratio r (also referred to as the stoichiometric imbalance; [donor]<[acceptor]) is chosen such that at full conversion of the Michael donor groups, the value of α>1, then gelation will not occur. For example, the ratio of acrylate to acetoacetate groups is generally required to be >2.5/1 for mono acetoacetates (difunctional) and triacrylates. Since the reaction is exothermic, it is usually required to slowly add the stoichiometrically deficient amount of donor species to a solution of the acceptor and catalyst at a rate that prevents a runaway thermal reaction. It is also required to ensure that very efficient mixing takes place during the addition to prevent the build-up of local high concentrations of the donor species, which would result in the formation of gel particles. At the initial stages of the reaction few difficulties are encountered, but as the oligomerization reaction proceeds, the viscosity increases significantly and efficient mixing becomes very difficult to achieve.
U.S. Patent Application Publication No. 2004/0029991 discloses UV curable acrylate terminated oligomers such as polyester acrylates, acrylate capped epoxy oligomers, polyacrylic acrylates, acrylate capped polyether polyols, acrylate terminated urethane oligomers or mixtures thereof. The acrylate terminated oligomers are prepared by first reacting a polyisocyanate with an acrylate having an isocyanate reactive group and then reacting the acrylate-isocyanate reaction product with an excess of polyol. The reaction conditions are held until no isocyanate functionality is detectable by titration in the reaction mixture. The acrylate capped urethane oligomer is then reacted with a Michael addition donor to form the UV curable oligomeric adhesive composition. The reaction is carried out in the presence of a base capable of promoting the Michael Addition reaction. On completion of the reaction, an acid can be added to the product to minimize viscosity buildup over time thus improving shelf life. In the production of such urethane resins, hydrogen bonding can undesirably increase the viscosity of the resin and mixing difficulties and residual basic catalyst can promote undesirable urethane reactions that limit the shelf-life of the resin.
There has been an ongoing desire to find photoinitiated adhesive compositions having good impact strength and toughness, because photocure mechanisms are ordinarily more rapid than heat cure mechanisms and can avoid heat degradation of the overall device, part and/or substrate.